Viral Waiting List
Before launching a new product, set up a waiting list so that people can access a beta list or be among the first users. To join the list, they must sign-up by leaving their email address.
The higher on a person’s position on the list, the earlier they got access to the product. But to get higher in the list, they have to invite their friends to sign-up as well : the more of their friends join, the higher they get in the list.